1. Field of the Invention
The present invention relates to a process for preparing a fermented soybean milk.
2. Description of the Related Art
Recently, vegetable protein has attracted public attention from the point of view of health. Especially, soybean milk, which is made from soybean, containing good quality protein but not cholesterol, has attracted a great deal of public attention as a health food.
However, there are some problems in using the soybean milk. For example, the soybean milk has an unpalatable taste such as grassy smelling and stimulating taste due to minor components from soybean such as 2-hexenal and several saponins.
In order to improve the flavor of soybean milk, some methods including a step of fermenting soybean milk with lactic acid bacterium have been proposed, for example, in Japanese Laid Open Patent Applications Nos. SHO 61-141840, SHO 62-205735, SHO 63-7743, SHO 63-276979, HEI 2-167044, HEI 6-276979 and HEI 8-66161. However, the products obtained by these known methods still have some problems such as insufficient masking of the unfavorable soybean flavor, unpleasant fermentation flavor, and a disappearance of the soybean""s body and taste.
For the purpose of solving these problems, the inventors have tried to ferment soybean milk with various strains of lactic acid bacteria, and have found that lactic acid bacterium of the genus Bifidobacterium is useful to improve the unpleasant flavor peculiar to soybean without adding components other than soybean. However, when the inventors Lactobacillus bulgaricus, which had been known to have flavor improving ability, in addition to Bifidobacterium bacteria, the former bacteria could not grow well in soybean milk. When saccharides that are utilized by lactic-acid bacteria were added to the soybean milk, Lactobacillus bulgaricus could grow well in soybean milk, but the flavor of the obtained fermented soybean milk was not improved sufficiently.
On the other hand, the conventional lactic-acid fermented soybean milk has a problem that it generates reverse odor of soybean during cold storage. Therefore, conventional products are heat-treated so that the bacteria in the products are destroyed. However, the heat treatment causes aggregation of the product or generation of unpleasant taste.
One object of the present invention is to provide lactic-acid fermented soybean milk having an improved total flavor without the unpleasant flavor of conventional soybean milk.
The present invention provides a process for preparing fermented soybean milk comprising the steps of: inoculating a lactic acid bacterium of the genus Bifidobacterium, Lactobacillus bulgaricus and one strain selected from the group consisting of Lactobacillus acidophilus and Lactobacillus casei to soybean milk, and fermenting the same.
Soybean milk used in the present invention is preferably one obtained by the process comprising the steps of contacting dehulled and dehypocotyl whole soybeans with warm or hot water; removing the warm or hot water-soluble component from the soybeans; pulverizing the soybeans to make a slurry, and removing the insoluble component from the slurry.
Another object of the present invention is to provide a lactic-acid fermented soybean milk, which can be stored longer without the problem of reversed odor of soybean. The present invention also provides a process for producing lactic acid fermented soybean milk further comprising the step of introducing the lactic acid fermented soybean milk into an container made of a material having an oxygen permeability of below 100 cc/ m2/24 hr/atm (25 xcexcm, 25xc2x0 C., 50% RH).
In the present invention, soybean milk may be one obtained from whole or defatted soybeans according to any conventional method and may be any conventional commercially available soybean milk. For example, soybean milk can be obtained by the process comprising the steps of impregnating whole soybeans or dehulled soybeans with water by immersing them into water or any other procedure, pulverizing the same with water to make a slurry and removing the insoluble portion of the slurry by a method such as filtration, to provide soybean milk.
The soybeans employed in the present invention may be whole or defatted soybeans, preferably dehulled soybeans, and more preferably dehulled and dehypocotyl soybeans. According to the present invention, it is preferred to remove the hot or warm water-soluble portion of the soybeans by contacting the same with the hot or warm water and then separating and removing the water phase containing the portion eluted in the water. Said portion may contain isoflavone, sapogenin and saccharides. In the case of whole soybeans, the soybeans may be contacted with water at a temperature of about 50 to 100xc2x0 C. for about 30 minutes to 10 hours. In the case of dehulled and dehypocotyl soybeans, the temperature of the water may be about 20 to 100xc2x0 C. and the contacting time may be about 20 minutes to 10 hours. Most preferably, the hot or warm water-soluble portion may be removed from the soybeans so that said portion remaining in the soybean milk is no more than 20 percent, preferably, no more than 15 percent by weight of the dried solid portion of the obtained soybean milk.
According to the present invention, the pH of the hot or warm water with which the soybeans are contacted may be from neutral to weakly alkaline.
Preferably, the pH is weakly alkaline in order to avoid stimulating taste due to the effect of xcex2-glucosidase. Any food additive, which can adjust the pH to weakly alkaline, may be employed to control the pH. For example, a salt of an organic acid such as sodium dioxide carbonate and sodium carbonate is preferably used from the point of view of flavor.
After that, the treated soybeans are pulverized in water to make a slurry and then, soybean milk can be obtained by separating and removing the insoluble portion from the slurry.
The step of pulverizing the soybeans may be conducted at room temperature. However, in order to prevent degradation of the product during the pulverizing step due to lipoxygenase, peroxidase or xcex2-glucosidase, it is preferable to carry out the step at a temperature of below about 10xc2x0 C. or above about 80xc2x0 C.
The obtained slurry may be kept above about 80xc2x0 C. and then may be separated into okara and soybean milk by any conventional means such as filtration, decanting, centrifugation or the like.
The soybean milk obtained as above may be sterilized by, for example, heating it to about 135 to 150xc2x0 C. for about 1 to 120 seconds before fermentation.
Since the soybean milk obtained as above has good flavor, it will provide good flavor to the fermented soybean milk of the present invention. In addition, because of the relatively small concentration of saccharides, it is easy to control the fermentation of the soybean milk by adding saccharides to the same, which saccharides are utilized by lactic-acid bacteria.
It is preferable to add some saccharides, which are utilized by lactic-acid bacteria, to the soybean milk to promote fermentation. Any saccharides utilized by lactic-acid bacterium may be employed, and oligo saccharides are preferable. When oligo saccharides are employed, they may be added to the soybean milk in an amount of about 0 to 5%, preferably, 1 to 2% by weight of the soybean milk. In addition, bifidus factor may be added to the soybean milk.
The lactic-acid bacterium of the genus Bifidobacterium used in the present invention may be any strain belonging to said genus. For example, B. bifzdum, B. longum, B. breve, B. infantis, and B. animalis may be used for the present invention. However, from the point of view of health, it is preferable to use bacterium occurring in human, i.e. other than B. animals. 
Lactobacillus bulgaricus used in the present invention may be any of the known strain and these are commercially available.
Both of Lactobacillus acidophilus and Lactobacillus casei used in the present invention may be any of the known strains and these are commercially available.
It has been known that Lactobacillus bulgaricus grows very well in cow milk but not in soybean milk. Therefore, when it was inoculated to soybean milk together with lactic acid-bacteria of the genus Bifidobacterium, it could not grow well enough and therefore, could not improve the flavor. The inventors however found that, when either Lactobacillus acidophilus or Lactobacillus casei is inoculated into the soybean milk in addition to these other two bacteria, Lactobacillus bulgaricus grew well in the soybean milk and improved the flavor of the resulting fermented soybean milk.
In general, soybean milk fermented with a lactic-acid bacterium of the genus Bifidobacterium alone contains unfavorable components produced during fermentation, such as acetic acid and diacetyl, which generate a cheese like odor. According to the present invention, such unpleasant odor or flavor are masked or reduced by culturing the soybean milk with the combination of these three bacteria, and a pleasant fermenting flavor may be generated.
These three bacteria may be inoculated all together to the soybean milk for lactic-acid fermentation, or the soybean milk may be fermented with the respective bacterium separately and then mixed together. It is preferred to inoculate them together.
According to the present invention, these bacteria may be purely cultured before inoculation to form so to speak xe2x80x9cbulkxe2x80x9d starters. Alternatively, freeze-dried bacteria or freeze-dried concentrated bacteria may be inoculated directly into the soybean milk.
The dosage of the inoculation may vary depending on the temperature or time of the fermentation step. For example, the total amount of bulk starters inoculated to the soybean milk may be about 0.5 to 15%. The freeze-dried bacteria may be inoculated so that the starting concentration of total lactic-acid bacteria is above about 105 cells/ml.
Then, the inoculated soybean milk may be fermented at about 20 to 50xc2x0 C., for about 3 to 48 hours, preferably at about 25 to 45xc2x0 C. for about 4 to 24 hours. Fermentation may be carried out with any conventional device or fermentation tank used for producing fermented cow milk.
In one embodiment of the present invention, the soybean milk may be fermented under the condition that the fermentation atmosphere is substantially free of oxygen, that the oxygen concentration of the atmosphere is lower than 5.0%, preferably lower than 3.0%. By fermenting under such a low oxygen atmosphere, the fermented soybean milk of this embodiment may be free from unfavorable odor peculiar to soybeans, and can obtain a body like that of cow milk and a good flavor.
In order to adjust the oxygen concentration during fermentation, the soybean milk inoculated with the lactic-acid bacteria may be introduced into an airtight container and fermented therein so that there is no room for air. Alternatively, the oxygen concentration of the atmosphere, such as the headspace of the container or fermentation tank, may be reduced.
In order to reduce the oxygen concentration of the atmosphere, deoxygenating agent, evacuation procedure or gas exchanging procedure may be employed and the gas exchanging procedure is most preferable. In the present invention, the gas used for exchanging the atmosphere is not limited. For example, an inert gas such as rare gas, nitrogen gas and carbon dioxide can be used for this procedure. Nitrogen gas and carbon dioxide are preferable because they are safe for food.
During fermentation, the concentration of oxygen dissolved in the soybean milk is not so important as the oxygen concentration of the atmosphere, and may vary depending on the temperature of the soybean milk or the pressure of the atmosphere. Preferably, the concentration is below about 2.0 ppm. The lower the oxygen concentration of the soybean milk is, the better will be flavor of the product. In order to reduce the amount of oxygen dissolved in the soybean milk, a vacuum deodorization machine such as a vacuum chamber may be employed.
The obtained fermented soybean milk of the present invention may be stirred and cooled, and then introduced into a container to provide a soft type yogurt-like product, or homogenized and cooled to provide a yogurt-like drink product. The products obtained according to the present invention can be combined with any conventional flavor, sweetener, colorant and stabilizing agent as desired. In addition, the products can be combined with a fruit preparation to provide a fruit type product.
In one embodiment of the present invention, the fermented soybean milk is stored in a container having oxygen permeability of less than 100 ml/m2/24 hr/atom (at 25 xcexcm, 25xc2x0 C., 50% RH), preferably, less than 10 cc/m2/24 hr/atom (at 25 xcexcm, 25xc2x0 C., 50% RH). Due to the low permeability of the container, the lactic-acid fermented soybean milk can be protected from contacting outside oxygen and, therefore, the product can be protected from the generation of reverse odor of soybeans.
The container having such a low oxygen permeability may be made from materials such as glass (less than 0.1) or resins. Examples of resins include polyvinyl chloride (PVC)(1), poly vinylidene chloride (PVDC)(1), ethylene-vinyl-alcohol copolymer (EVOH)(2), polyethylene terephtalate coated with polyvinylidene chloride (K-PET)(20), ceramic vacuum evaporated PET (0.2), and aluminum vacuum evaporated PET (0.2) (each number after the name represents the respective oxygen permeability rate cc/m2/24 hr/atm (at 25 xcexcm, 25xc2x0 C., 50% RH)). The container may be any form as long as made of the materials listed above. It may be a container made from glass, plastic resin, plastic resin+paper plastic film, paper plastic film, metal such as aluminum, and metal+plastic film. The lid of the container should also be made from one of the above listed material. From the environmental point of view, however, some of these materials have disadvantages for example, used glass containers are difficult to collect and aluminum vacuum evaporation requires a high burning energy. The most preferable container is made from a combination of paper and plastic film having oxygen barrier property (so called multi layer structure). Examples of a plastic film having oxygen barrier property include ceramic vacuum evaporated PET (0.2 cc/m2/24 hr/ atm), EVOH-PET (2 cc/m2/24 hr/atm), K-coating PET (20 cc/m2/24 hr/atm) but are not limited to them.
According to the present invention, the soybean milk inoculated with the specified lactic-acid bacteria may be introduced into the above container, and fermented and stored. Alternatively, the inoculated soybean milk may be fermented in a fermentation tank and then the product may be added to the container.
In this embodiment, the good flavor of the lactic-acid bacteria fermented soybean milk can be kept over 2 weeks when stored in the cold, at a temperature between about 0 and 10xc2x0 C.
In the present invention, the lactic-acid bacteria contained in the fermented soybean milk should be active and the concentration of the active bacteria is preferably more than 1 million cells per 1 ml. In order to keep the good flavor of the lactic-acid fermented soybean milk longer, the concentration of bacteria other than lactic acid bacteria is preferably less than 10 cells per 1 ml. To eliminate contamination of the bacteria, it is preferable to add the lactic-acid fermented soybean milk to the container specified as above under a germ free condition, for example with the use of an aseptic filling up device.
It is preferable to fill up the container with the fermented soybean milk in order that the fermented soybean milk is prevented from contacting oxygen. In case there is some headspace in the container, the space is preferably filled with an inert gas for example, nitrogen gas.